use std::{
    convert::TryFrom,
    io::{stdin, BufRead},
};

use cmpmore::CmpMore;

fn main() {
    let stdin = stdin();
    let mut stdin = stdin.lock().lines().map(Result::unwrap);
    let [n, d] = <[_; 2]>::try_from(
        stdin
            .next()
            .unwrap()
            .split_whitespace()
            .map(|x| x.parse::<usize>().unwrap())
            .collect::<Vec<_>>()
            .as_slice(),
    )
    .unwrap();
    let d = d as i64;
    let mut dp = [0, -d];
    for _ in 0..n {
        let w = <[_; 2]>::try_from(
            stdin
                .next()
                .unwrap()
                .split_whitespace()
                .map(|x| x.parse::<i64>().unwrap())
                .collect::<Vec<_>>()
                .as_slice(),
        )
        .unwrap();
        let mut swp = [0; 2];
        for i in 0..2 {
            for j in 0..2 {
                swp[j].change_max(dp[i] + w[j] - (i != j) as i64 * d);
            }
        }
        dp = swp;
    }
    let ans = *dp.iter().max().unwrap();
    println!("{}", ans);
}

// cmpmore {{{
#[allow(dead_code)]
mod cmpmore {
    pub fn change_min<T: PartialOrd>(lhs: &mut T, rhs: T) {
        if *lhs > rhs {
            *lhs = rhs;
        }
    }
    pub fn change_max<T: PartialOrd>(lhs: &mut T, rhs: T) {
        if *lhs < rhs {
            *lhs = rhs;
        }
    }
    #[macro_export]
    macro_rules! change_min {
        ($lhs:expr, $rhs:expr) => {
            let rhs = $rhs;
            let lhs = $lhs;
            $crate::cmpmore::change_min(lhs, rhs);
        };
    }
    #[macro_export]
    macro_rules! change_max {
        ($lhs:expr, $rhs:expr) => {
            let rhs = $rhs;
            let lhs = $lhs;
            $crate::cmpmore::change_max(lhs, rhs);
        };
    }
    pub trait CmpMore: PartialOrd + Sized {
        fn change_min(&mut self, rhs: Self) {
            change_min(self, rhs)
        }
        fn change_max(&mut self, rhs: Self) {
            change_max(self, rhs)
        }
    }
    impl<T: PartialOrd + Sized> CmpMore for T {}
}
// }}}